The production of nitrogen from air and other nitrogen and oxygen containing gas mixtures can be performed in a number of industrially suitable techniques including cryogenic distillation, membrane permeation and adsorption, using pressure swing adsorption or temperature swing adsorption techniques. Recent mid-range volume requirements for nitrogen for industrial end uses and other end uses have been successfully met by the use of pressure swing adsorption techniques. Competition to provide pressure swing adsorption systems having the simplest equipment arrangement, the smallest size and the least cost in both procurement and operation has been experienced in the industrial gas industry directed to nitrogen products.
However, the nitrogen producing industrial gas industry is still beset with the problem of producing dry, nitrogen-enriched product from effectively wet and carbon dioxide-containing feed gases, such as ambient air.
The presence of water in air has a negative effect on the performance of air separation adsorbents. The water reduces both gas uptake rates and capacities which serve to lower the production capability of the adsorbent. Water can also cause oxidation of carbon molecular sieves which further reduces gas uptake rates. Given the negative impact of water, two techniques are currently used to remove moisture prior to the air separation adsorbent. These techniques include (1) water removal by chilling and condensing or (2) gas drying with conventional desiccants.
Water can be effectively removed from gas streams by cooling the stream, condensing out the water and heating the stream back up to reduce its relative humidity. This technique reduces the relative humidity of the inlet air stream which minimizes the deleterious effects of water on adsorbent performance. Typically, this procedure is carried out by passing the air through a refrigerant chiller which reduces the gas temperature and condenses much of the inlet moisture. The resultant low relative humidity air (after gas heat-up) is then sent directly to the adsorptive separation.
The shortcomings of this technique of water removal are fairly evident. Firstly, the chiller adds a piece of equipment to the process design and as such increases the plant capital cost. In addition, the presence of the chiller increases the energy usage of the system and thereby increases power costs. Finally, refrigerant chillers tend to be high maintenance items and will lead to system downtime.
Conventional desiccants include inorganic species like zeolites, aluminas and silica gels. These materials are used as desiccants because they have high water adsorption capacities and favorable water adsorption isotherm shapes. The water adsorption capacity of these materials varies from 20 to 50 wt %. This high capacity limits the adsorbent requirement for drying. These materials also have water adsorption isotherms that are concave to the pressure axis, particularly at low pressure, which helps in forming short, sharp mass transfer zones. Thus, conventional desiccants have water adsorption properties which minimize the amount of adsorbent needed to dry gas streams.
However, these conventional desiccants are all polar materials. Because these conventional desiccants are polar, they selectively adsorb polar molecules like water. With respect to the major components of air, these adsorbents, particularly the zeolites, show selective adsorption of nitrogen over oxygen due to the more polar nature of nitrogen. This is a clearly undesired situation with respect to nitrogen production by pressure swing adsorption (N.sub.2 PSA). Thus, for N.sub.2 PSA applications conventional, polar desiccants have the undesired property of N.sub.2 selective adsorption. The use of conventional desiccants (alumina) is the technique currently employed in many N.sub.2 PSA processes.
Thus, both previous solutions to the problem of water removal for nitrogen pressure swing adsorption processes based on oxygen selective adsorbents have shortcomings. Conventional desiccants display nitrogen selective adsorption, which is undesirable. Even in cases where the nitrogen selectivity is minimal, conventional desiccants will act as unselective sections in the nitrogen pressure swing adsorption beds thereby lowering nitrogen recovery and productivity. Water removal with refrigerant chillers has the drawbacks of increasing both capital and power costs as well as adding high maintenance equipment to the installation.
U.S. Pat. No. 3,923,477 discloses a pressure swing adsorption system having drying beds that precede the beds which selectively extract oxygen from air to produce a nitrogen-enriched product. At column 2, lines 8 through 10 the patent mentions that separate beds are unnecessary when recovering a nitrogen product from air.
Such compound adsorption beds containing a desiccant layer and a main adsorbent layer selected for the primary separation are illustrated in U.S. Pat. No. 4,326,858.
More specifically, in British Patent 2,042,365 adsorption beds having a desiccant layer followed by carbon molecular sieve for the selective adsorption of oxygen preferentially over nitrogen in an air separation process resulting in nitrogen enriched product is described. The desiccant is identified as alumina or silica gel.
Russian Patent 1,219,122 discloses a composition for drying gases using adsorption technology in which the composition includes activated aluminum oxide, activated carbon, a binder and a hygroscopic additive of lithium bromide. The only recited utility of the composition is as a sorbent of moisture intended for the drying of gas-air media.
U.S. Pat. No. 4,677,096 discloses activated carbon which is impregnated with various agents selective for diverse gases generally considered to be toxic to human breathing, other than moisture.
U.S. Pat. No. 4,708,853 discloses carbon molecular sieves which are impregnated with various agents which are selective to the adsorption of mercury from gas streams.
U.S. Pat. No. 4,402,717 discloses a dehumidifying and deodorizing system which impregnates activated carbon on a paper substrate and further impregnates the activated carbon with desiccants such as lithium bromide, lithium chloride, potassium chloride, etc.
U.S. Pat. No. 4,702,749 discloses the treatment of activated carbons in an oxidizing acid wash to introduce surface oxide groups which makes the carbon relatively more hydrophylic. These activated carbons are then used in adsorptive drying.
The literature article, Activated Carbon Adsorbent For PSA Driers, T. C. Golden, et al., Carbon, Vol. 28, No. 5, pp. 683-690 discloses a process for oxidizing the surface of activated carbon to render it hydrophylic for water adsorption.
Therefore, the problem facing the industrial gas industry is the more effective removal of water from ambient air and thereby the enhancement of the performance of a nitrogen-enriched gas generating pressure swing adsorption process. This requires a desiccant material that demonstrates oxygen selective adsorption. There are current desiccant materials that display nitrogen selective adsorption such as zeolites, however oxygen selective desiccants are not known and prior to the present invention this problem remained unsolved.
The prior art although implementing desiccants as pretreatments in pressure swing adsorption systems, has failed to provide an appropriate advantage in simplified, compact systems for the production of nitrogen-enriched gas products in an efficient manner. The present invention overcomes the drawback of the prior art, as set forth below.